Method and apparatus for electric arc welding



April 22 1958 R. B. STEELE METHOD AND APPARATUS FOR ELECTRIC ARC WELDING Filed Oct. 8, 1954 2 Sheets-Sheei 1 April 22, 1958 E. E. ETEEEE 2,832,000

METHOD AND APPARATUS FOR ELECTRIC ARC WELDING Filed Oct.- 8, 1954 2 Sheets-Sheet 2 INVENTOR. RICHARD B. STE ELE -BY H ATTORNEY 1 2,832,000, a METHOD AND APPARATUS FOR ELECTRIC ARCWELDING 4 a Richard B. Steele, New ProvidencepN. 1. as signor to Air f Reduction Company, Incorporated, New York, N. Y., a corporation of New York.

- 2 Application October a, 1954, Serial No. 461,065 l m-] C -13 9)L t T i Thisinventionrelates to electric 1 arc weldin'g methods and apparatus and rnore particularly to methods and appar'atus for controlling the position of a welding electrode for automatic electric ar'c welding 1 "Automatic arc welding heads are now well;lgnown in the art and they generallyprovide an electric motor driven. means for advancingand retr acting; the end of a welding electrode toward and awa y; from a workpiece tofbe welded in such a mannerfas to establish an -arc from theelectrode to the workpiece and thereatter to maintain the spacing of the end of the electrode from the; work in a manner to maintain the are while the elec; trode and workpiece are movedwithrespect to one an:

other to thus form a weld. Welding-headsof this type;

have been provided for use with consuming eleetrodes and with non-consuming electrodes. In genepalthe utility. of such heads is limited 'by the. speedlofnespoiise to variations in arc length which is in'part li ited. by the ou putrs c e djus ss rk ttt q motors 1 .o 'fp' d ad u t P e and sp sdio e ns if a ame p a c inthe asttq o -two mo q iwhich. ui n ou a P v e-th r t i na in t; ote differential, the difierenceoutput of the, ditferelitial being s d o pos n e e e e, Suc sys ems are-rela-v tively costly, heavy, and bulky. a;

object of the present invention is .tO provide. novel h s and ap a t v m o si e m m qn rbl, w e ua e power a p e ot r sp nse the. positioning of-a welding electrode with respect to a work-.

piece during welding. v11. 1 Another object of the inv nt- 88 552110. provide novel.

expendable fusible elements, and the disadvantage of extended high frequency dischargeflcreatinginterference with communications; By the present invention the electrode lS advanced to. touch the worklwith the electrode substantially de-energized' and then, simultaneouslywith the retraction, of theelectrodethe' Welding current is i turned on and a high frequencyldischarge isapplied to the electrode. This provides positive arestarting with no possi bility of electrode sticking or electrode contamination and it eliminates the need .for fusible plugs. Further the present inventionprovides .for automatically de energiz ing the high frequency source as soon' as the arc estab' lished, thus reducing interference with communications to. a minimum. A

In general'this' invention contemplateshtheuse'of' atwo phase induction motor of conventional design, which islcharaeterized -by its high torque and fast response, to

advance and retract 'a welding electrode with respect to the work to be weldedto establish andmaintain a weld-j ing-arc from the electrode tothe work having a substantially constant preselected arc voltage. To efiect this control of, the motor, one field winding is energized byalternating current obtained directly from the service line but phaseshiftedapproximately 90 by a series ca paciton, ,The other field winding is provided with alternating current or pulsating direct currentfrom thesame service line but controlled by a .D.3C;; circuit which're sponds towelding arc voltage. The net current applied to this second motor field winding is veitherinphase with or 180 out of.phasefwith"the service line which makes the energization of this second field Winding either lagging or leading the energizationot the first'field'windingby.

about 90; This phase relationship determines the direction of rotation of the motor. The speed of rotation is' determined by the net valueof the.-,current in the second.

' motor field winding, Thelcontrol circuit is adirect curmethods and apparatus for controlling the; direction -and.

speed of rotation, of atwo phase induction motor. 1

other o i s to p ov de novel metho s d app head having a novel automatic arc starting-system utilizv sa h ,f equ y s s ha iu- -2 .5 so Thes a d. o he O t dt v m g sp the tion will be pointed out or will beeome apparent low n ed st n es ne mblod m ntd i h, invention shownfor purposesgof illustration in the ac companying. schematic 'drawingl I Automatic fare 'weldin hefdsj of the" established thewelding fa cia one pr three 1w,

theyfadvanced the energized enamelsthatitjto heai;

thework and then retracted itjto form the arc, advan d the energized electrode into contact' with 'a meannes suchas' a steel wool ball between: the endof the'elect rodejf and th'e work, or started the' are by adya'ncin'gjthe clog-I trodeuntil the arcgapwa's bridged'byiafhigh fre: :e'ncy;

st re a mcohs'istencyihs gjeirpense i rent circuit; which comparesthe arc-voltage with a preselected constant voltage ,anduses; the amplified difference voltageto control grid controlled rectifiers-in a full wave rectifier circuit. The second -motor :winding is in the common leg. of the fullwa-vesrectifier circuit. To increase the torque Ofthe motor a; capacitor is provided acrossthe second motor; field winding tozprovide reverse current fl w (m ed ;A- C-Mmt nd g: atthe pp u e, timeduring periods of desired maximum motor energization.

.Aby-pass of one of the grid controlled rectifiers, which by-pass includes a-current limitingresistor, provides fixed energization of the control motor in a direction opposing the mechanical-'biasing efiect..of gravity or other fixed mechanical bias applied t o-the apparatus. 7

The invention also; contemplates a novel combination of electrical control-elements; which automatically connects thesecondtield winding of the motor across a direct current supply eachtimethe motor is de-energized, to

thereby eifectively dynamically brake the motor and insure an,instantaneous-stop. Y H

l?he ,novel,control circuit ofthis :invention also provides for starting the are. with the aid of a high-frequency dischargeof; minimum; duratiomwhich effectively minirnizes interference with communications from this source.

For a complete and thorough,understanding of this invention reference shouldbe made to the following description and the accompanying drawings.

Fig. 1 illustrates schematically. both the electrical and mechanical elements constituting the improved welding head forIweld-ing withahon-consumable electrode.

. fi pns ati s po i io Figs 2 illustrates asniall portion of the apparatus of Fig. 'modified to feedj 'Qconsumable electrode.

' X i ding head of Fig.1 in a hori- F 3 illustrates .th

Referring to" Fig." 1' the workpieces to be welded .3 designated. as 10' and 11 and are shown supported by a motorized carriagelS in operative position with respect to an arc welding electrode 12 supported by an electrode holder 13. In the form illustrated, the electrode may be a, tungsten electrodewhich is substantially non-consumable and which is surrounded by a flowing stream of inert shielding gas in the now familiar inert gas shielded non-consuming electrode arc welding process. In that instance, the electrode..12 is tightly gripped in the electrode holder which in turn is raised and lowered by appropriate gears, such as the rack 14 and the engaging pinion 16 which acts to raise and lower the entire electrode/holder and thus the electrode 12 with respect to thework. The invention is equally applicable, as shown in Fig. 2 to a consuming wire electrode which melts off in the heat of, the arc and is transferred across the arc to deposit filler metal on the. work. A method and apparatus for welding of this type is disclosed in Muller et al. Patent Number 2,504,868 issued April 18, 1950. In sucha Welding system nip rollers 16 engage the wire 12 directly to support it and feed it through the welding head 13.. Where the present invention is applied to this type welding the nip rollers are driven by the same mechanism to be hereinafter described that drives the pinion gear 16 in the form of the invention illustrated in Fig. l. The pinion gear 16 is rotated by shaft 17 driven by wormwheel 18. The mechanical rotation of worm wheel 18 is derived from the rotation of worm 19 and from limited axial translation of worm 19 while it is in engagement with the worm wheel. The rotation of worm 19 results from the rotation of armature 21 of a motor M transmitted through shafts 22 and 23 and the inter-connecting speedreducing worm gear set 24, 26. It is obvious from this mechanical schematic drawing that rotation of motor armature 21 in one direction will cause the electrode holder 13, and hence the electrode 12, to be moved in a direction away from the work, whereas rota tion of the motor armature 21 in the opposite direction will cause the electrode tobe moved toward the work.

- Shaft 23 is splined to permit axial movement of worm 19 along the shaft without permitting relative rotation between the worm and the shaft. Collar 27 limits the movement of the worm along the shaft in one direction. A bifurcated crank 28 having a fixed pivot point 29 straddles the shaft 23 in such a manner as to bear against the worm 19 through a thrust bearing 25 and in so doing limits the axial displacement of worm 19 along the shaft 23 in the other direction. With the orientation of the apparatus as schematically illustrated, gravitational force tends to rotate pinion gear 16 in a clockwise direction.

This in turn tends to rotate worm wheel 18 in a clockwise direction and the worm gear 19 advances to the left until it comes to rest against the thrust bearing 25 which in turn rests against the bifurcated crank 28. An are starting solenoid having a magnetizing winding 31, has its armature 32 connected through link 33 to the crank 28. When this solenoid is de-energized, the armature drops down which is in the same direction as the force exerted on the armature through the crank by the weight of the electrode holder 13. When the winding 31 of the solenoid is energized, as will be fully explained hereinafter, the armature 32 pulls in, rotating the crank 28 about its pivot point 29 in a clockwise direction, which slides the worm 19 along the shaft 23 until it comes to rest against the collar 27. This action rotates the worm gear 18 in a counter-clockwise direction which acts to raise the electrode away from the work.

Motor M, is a two-phase induction motor of a conventional design commonly employed in servo-mechanisms. This type motor is characterized by its high torque and fast response. It comprises two field windings 34 and 36, which are independently energized by alternating current, and are approximately 90 out of phase. Thus, if the current in winding '36 leads the current in winding 34, theyarmature 21 will rotate in a given; direc- Cal tion. If the current in winding 36 lags the current in winding 34, the armature 21 will rotate in the opposite direction. Current is supplied to winding 34 from the A. C. supply through phase shifting capacitor 35, as will be more fully described hereinafter. Welding current is supplied from a welding machine 37 to the work 10 and the electrode holder 13 (and hence the electrode 12) through the conductors illustrated in the diagram by heavily weighted lines.

A power actuated switch, hereinafter referred to as the welding contactor, has an actuating coil 38 and normally open contacts 38a in the welding circuit. The secondary winding 39 of a radio frequency transformer 41 is also included in the welding circuit for the purpose of introducing into the welding circuit a high frequency high voltage discharge to assist in are starting. The primary winding 42 of coupling transformer 41 is energized from a high frequency oscillator 43. As is evident from the electrical circuit the voltage appearing across the welding arc gap also appears across the terminals of a double pole doublethrow switch 44,- which permits the arc voltage to be applied to the control circuit with proper polarity regardless of the polarity of the welding operation. Inductance 46 andcapacitors 47, 48, 49, 51 and 52' act to filter out from the control circuit and from the welding machine any alternating current component incidental to the-application of the high frequency to the welding circuit. Voltmeter 53-is preferably provided in a convenient location to enable the, operator to observe the arc voltage at all'tirnes. A filter network consisting of the inductance 54 and the capacitor 56effec'tively eliminates any low frequency ripple characteristic of the welding current produced by the welding machine 37. Such filtering is particularly necessary with a welding machine of the rectifier type. By virtue of the circuit just described are voltage appears across resistor 72.

A power's'upply of conventional design is provided as a source of direct current for the D. C. control circuit to be hereinafter described; It is preferably a full wave rectifier having adequate filtering. In-the schematic diagram a full wave high vacuum rectifier tube 58 is appropriately connected in circuit with thepower supply transformer 59 to produce a full wave rectified output which is-filtered toa substantially smooth direct'current by the filter network comprising'inductance 61 and capacitors 62 and 63; A voltage dividing circuit including fixed resistors 64 and 66-andthe variable control resistance 67 (are voltage control) is connected across the output of the powersupplyr The voltage regulator tube 68 assures a constant voltage across that part of the voltage dividing circuit to which it is connected. A stable D. C. reference voltage is-therefore established between fixed resistance 66 and the variable resistance 67. This voltage is applied throughmesistor 69 to the control grid of one Seetion- 71a-of adual triodet71. Arc voltage'which appears across resistor 72 as described above is applied to the control grid of the other half 71b of-the dual t'riode 71.- A common'cathode bias resistor 73 is-providedin the cathode circuit. Capacitors 74 and 76act to filter out transientfluctuations from the voltages applied to the control grids of -tube 71. The usual load resistors 77 and 78mm respective plate circuits of tube 71 are provid-ed. Because of the common cathode resistor 73 an increase in current in tubev 71b caused by an increase in arc voltageresults in a decrease in current through tube 71a. The amplifiedlvoltage difference between arc voltageand the reference voltage as it appears at the plates of tube 71 .is applied to a differentiating and integrating circuit of conventional design which effectively prevents hunting. Resistors 79, 81 and 82, and capacitors 83, 84.

and 86 constitute the.-anti-hunt network. The stabilized voltage output of, the anti-hunt circuit is applied to the gridsof another dual triode amplifier circuit whichin r l, and load resistors 89;and-91 inthe plate circuits. sVariable resistors 88 actto regulate'theamplification (sensitivity) of, the control circuit. The output voltages taken from the plate circuits of the amplifier tube 90 are applied to the control. grids of tubes 92 and 93. These tubes are connectedlas grid controlledrectifiers in a full wave rectifier circuit including the powertransformer 94 having aprimary winding 95 and a main secondary winding 96 connected throughresistors 97 and 98 to the plates of tubes 92 land 93 respectively. .The center tap of; the secondary winding. is connected to the cathodes of tubes 92 and 93 through the motor field winding 36 so that it, passes pulsating direct current on each half cycle when both ,tubes 92 and 93 are conducting.

, When the arc voltage is at the proper preselected value there is an exact balance between the voltage applied to the respective controlgrids of tube 71. This exact balanceis maintained through the amplifier, and in the absence of any other circuitry, balancedfull wave rectified pulsating direct current'would pass through the:- motor field: winding 36. ;Sinc e,the current pulses in the field winding 36 on one, half cycle lead the current in winding 34 and exactly match the current pulses throughthe field winding 36 on the other half cycle which lag the, current in field winding '34, balanced conditions are obtained in the motor and no net rotation of the armature is produced. When; the: arc-voltage is above the preselected value, the control grid of tube 71b becomes more positive with respect to the cathode than does the control grid of tube 71a, and the voltage at the plate of 71b becomes lower than the.voltage at the plate of 71a. =This produces a proportionately-higher voltage at the plate of tube 90b.than;atthe plate of 90a. :This permits tube 93 to conduct more current than tube 92 resulting in an unbalance betweenthe half cycles in the. full wave rectified current flowing in motorfield winding 36 and the motor 1 rotates in a direction to move the electrode toward the work to shorten .the arc gap. Conversely, if the arc voltage is too low, the motor moves in adirection to lengthen the arcsgap. The control circuit components are 'prefe'n ably so selected that when the voltage between the grids of tube 71 is of the-order of'magnitude: of ,4 volt the appropriate tube of tubes 92 and 93 will be biased to cutofi andtheother of thesetubes will be at full conduction' permitting full power output from motor M.

To further increase the power output from the motor M a capacitor 99 maybe connected-in parallel with the motor field winding 36 to produce some reverse flow of current in the motor field'windingduring the non-conducting half cycle when oneof the tubes 92 or 93 is cut ofi. This. greatly increases the power output of themotor permitting use ofa: smaller motorthan-wouldotherwise berequired. w p

..In the particular embodiment ofrthe invention illustrated-the conductioneoftube 92 provides excitation of the motor field winding 36 in a phase relation to the excitationgotmotor, fieldwinding 34. such as to rotate the motor in adirection which raises the electrode holder 13 away fromt-he workpiece.- conversely produces rotation of the motor M in a direction to;lower--the"electrode holder 13. toward .t-he workpiece; In the preferred embodiment of the invention, a

by-pass'isprovided,around tube 92 through the current limiting resistor .101, permitting limited .energization of urotorain a dirpction to raise the electrode holder.

his comp n tes q the e ec 95 rav yi ntt ra,

r-atus When the apparatus is; used in a position other than-.the; gverticaL; such-gas the horizontal position-shown in Fig. 3 a spring 150 is preferably utilized to provide a bias force, tending to move the electrode holder .13 toward the work. In Fig. 3 the spring is in compression and acts 1 through lever. piv0 e abo t-p vo poin.t- 2, to urge thew d ng hea ow rd e Work- I .AIcapacitor 109 anda current'limi ting connected betweennherpositive sideot th xpq xens pply;

resistor 110, are.

6 and'the motor field winding 36. These elements' 'for'in part of the braking circuit for the motor which will be more fully described hereinafter. I a p i Power is suppliedto the entirecontrol circuit from an A. C. supply line, preferably volt 60 cycles, through a line switch 102 and protective fuses-103 to conductors 104 and 106. The primary windings of power transformers 59 and 94, the arc starter solenoid 31, the operating coil ofiwelding contactor 38, a power outlet'20 for. the motorized carriage '15, the high frequency oscillator 43, a solenoid valve107 to control theflow of shielding gas to the electrode holder, a solenoid valve108 to control the'tlow of cooling water to the electrode holder,

the motor field'winding 34 and the phases'hifting capaa citor35 in series therewith, and a'number of control relays to be hereinafter described in greater detail are all connected across the C. supply carried by conductors 104 and 106.

A control relay'lll'conne'cted' across supply conducof control relays 118 and 119. Control relay 112 con nected acrosssupply conductors 104 and 106 hasnormally open contacts 112a in the circuit of control relay 116, normally open contacts 112a and 112d in the control. grid circuits of tubes 92 and 93 respectively, and

normally closed contacts 112e and 112 connecting the control grids to the cathodes in tubes 92 and 93 respectively. Control relay 113 connected across the supply conductors 104 and 106 has normallyopen contacts 113a in its own circuit, normally open contacts 113b in the circuit of the arc starter solenoid 31, normally open-contacts 113d in the circuit of the welding contactor 38, and normally open contacts 113c in the circuit of the oscillator 43. Control relay 114 connectedacross supply'conduct-ors 104 and 106has normally close'dcontacts 114a-in series with current limiting resistor 101 in the circuit by-passing tube 92. Control relay 116 connected across supply conductors 104 and 106 has normally closed contacts 116a in the circuit connecting thecathode of tube 92 to its control grid, normally open contacts 1161a and 1160 in the cathode circuits of tubes 92 and 93 and normally closedcontacts 116d in a braking circuit connecting the rnotor field winding 36 to the negativeside of the D. C. power supply. Control relay-117 connected across supply conductors 104 and 106 has normally open contacts 117a in the circuit of motor field Winding 34, and normally'open contacts 117b in the cathode circuit of tube 92. Control relay 118 'is connected across the output of the welding machine and has normally open contacts 118a in the circuit of oscillator 43, and normally closed contacts 118b in the circuit of the motorized carriage power outlet 20. Control relay 119, connected across the welding contactor in the arc circuighas normally"opencontacts 119a in the circuit of control relay 113. Time delay relay 122 is connected in a circuit across the A. C. supply conductors 104 and 106 and has normally closed contacts 122a in the circuit supplying control relays 111 and 112. The contacts 131a of a fiow' switch 131 that responds to the flow water solenoid valves 107 and 108. A weld switch 132- in the circuit of. control relays 111 and 112 and of timedelayrelay 122gives thenecessary. control. for start.- ing and stopping the weld cycle.

Operation of the control circuit with the preferred method of are starting is as follows. The first step is to turn on welding machine 37 and close line switch 102. Closing line switch 102 immediately starts the flow of shielding gas and cooling water by energizing solenoid valves- 107 and 108 through the closed contacts 121a oftime delay relay 121. Time delay relay 121 is simultaneously energized through the normally closed contacts 111c of control relay 111. Energization of time delay relay 121 starts it timing but produces no immediate change in the condition of the contacts from their condition when the coil is de-energized; Also immediately uponclosing line switch 102 the are starting solenoid 31 becomes energized through contacts 111b, retracting the are starting solenoid armature 32, advancing. worm 19 to its extreme right hand position and raising the electrode holder. The closing of switch 102 also immediately energizes the primariesof power transformers 59 and 94, thereby warming up the-filaments of the vacuum tubes. After a time delay of the order of to 10 seconds, time delay relay 121 times out opening its normally closed contacts 121a which d e-energizes the solenoid valves and interrupts the flow of shielding gas and cooling water. This preliminary gas flow acts topurge the gas passages of any air that may have enteredthem by difiusion during an extended period of. inactivity. To weld, the operator closes weld switch 132 which immediately and directly energizes control relays 111 and 112 and time delay relay 122. Energization of control relay 111 acts throughthe openingof its contacts 1110 to de-energize time delay relay 121, resetting it and reinstating the contacts in the de-energized position. This results in the opening of solenoid valves 107 and M38 permitting shielding gas and cooling water to flow. If an inadequate supply of cooling water is fiowingthe water flow switch 131 acts to open contacts 131a, de-energizing time delay relay 122. In the event of inadequate water flow, time delay relay 122 remains energized and after a short delay its contacts 122a open, de-energizing control relays 111 and 112 thereby preventing welding and preventing damage to the equipment by virtue of the lack of adequate cooling. If the cooling water flow is adequate, time delay relay 122 is de-energized as described above, and control relays 1 11 and 112 remain energized. In addition to resetting time delay relay 121 as described, energization of control relay 111 causes its contacts 111e to close which puts control relay 118 across the Welding circuit open circuit voltage which voltage is sufiicient to actuate relay 118.. Relay 118 preferably picks up at about 45 volts and drops out at about 35 volts. Contacts 1181; close energizing control relay 114, opening its contacts 114a in the circuit bypassing tube 92. Contacts 118]; open preventing starting. of the motorized carriage when contacts 111 close. Control relay 111 when energized also causes control relay 117 to be placed across the line when contacts 111d close. This actuates the contacts of relay 117 in the motor control circuit. When control relay 111 is energized its contacts 1111; open de-energizing the arc starting solenoid 31 permitting the electrode holder to drop down into the ready position for are starting. Control relay 112 which is also energized when the weld switch 132 is closed closes its contacts 112-a energizing con trol relay 116 actuating its contacts in the motor control circuit. It canbe seen from this description that at this stage of the sequence of operation all of the contacts of control relays 112, 114, 117 and 116 in the motor control circuit assume the opposite position fro-m those shown in the drawing as these relays are now all energized. The effect of this is that the control grids of tubes 92 and 93 are directly connected through contacts 112c and 112d to the outputs of amplifiers 90a and 90b respectively. The by-pass circuit through resistor 101 around tube-92 is open, the circuits connecting the controlgrids. to the cathodes of tubes 92 and 93 respectively are-open, and the cathodes of tubes 92 and 93 are connected .to the positive output ofv the power supply through contacts 116b, 116e, and 1 17b. Contacts 1-16d'are open disconnecting the motor field winding 36 from the negative side of the D. C. power supply. Because the open circuit voltage of the welding machine greatly exceeds the reference voltage, the comparator and amplifier circuits associated with tubes 71 and hold-tube 92 at out off and cause maximum conduction through tube 93 in the form of pulsating D. C. resulting from the rectified output of transformer 94. Capacitor 99 charges during the conducting half cycle oftube 93 and discharges through motor field winding-36 on. the'non-conducting half cycle producing rnGdiiedA. C. in the winding, increasing the torque developed by the motor over that developed by pulsating D. C. excitation, and driving the electrode holder down toward the work.

Whenthe electrode 12,touches the workpiece, the open circuit voltage of the'welding'machine is applied to control relay 119energizing it and .closing its contacts 119a in the circuit of control relay 113. With contacts 111a of control relay 111 closed and with contacts 113a closed, control relay 113 is locked in across the line. Contacts113b also close energizing the arc starter solenoid 3 1 retracting the electrode to establish a-n arc gap. Simultaneously, contacts 113d close energizing weld contactor 38 closing its contacts 38!: placing the welding machine output across the'arc gap. When contacts 38a close control relay 119 is short circuited and its contacts 119a reopen. Also concurrently therewith, contacts 1130 close energizing the high frequency oscillator 43. The output of this oscillator is coupledto the welding circuit through transformer 41 and the high frequency is thus impressed across the arc gap, ionizing the arc gap and causing the welding arc to be established between the electrode 12 and the workpiece. As soon as the-arc is established the relatively low arc voltage is insufficient to maintain control relay 118 in the encrgizedposition and it drops out opening its contacts 118a which de-energizes the high frequency oscillator 43, as well as control relay 114. Contacts 118b also close energizing the external power-outlet 20 which-is preferably useclto supply power to the travel mechanism which advances the welding head with respect to the work. Deenergizationof control relay 114 causes its contacts 114a in the'circuit by-passing tube 92 to close, inserting the by-pass circuit and producing a minimum current in the circuit of motor field winding 36 having a phase relationship producing a torque tending to raise the electrode holder. Aspreviously described, this feature is intended to overcome the biasing effect of gravity on the apparatus. The-weld nowprogresses with all contacts in the positions indicated, and the motor control circuit acting to automatically maintain the arc voltage constant by constant automatic comparison of the are voltage with the fixed reference voltage. At the end of the weld, the operator opens weld switch 132 whichde-energizes control relays 111 and 112. De-energization of control relay 111 closes the contacts 111c and'starts the time delay relay 121 timing. Contacts 111d open de-energizing control relay 117 which opens contacts 117a in the circuit of motor field winding 34 and opens contacts 117b in the cathode circuit of tube 92. De-energization of control relay 111 also causes contacts lllf'to open, interrupting the power to receptacle 20 feeding the travel mechanism andalso interrupting the power ofthe weld contactor 38 opening its contacts 38a. Thus, it may e seenthe welding arc is interrupted and the travel of the-welding headwith respect to the work is simultaneously' interrupted. Opening the weldswitch 132 as described above also de-energizes the control relay 112 returning all its contacts to the de-energized condition.

' Thus, de-energ'ized control relay 112, opens its contacts in the control grid circuits of tubes 92 and 93, and closes the contacts connecting the control grids to the cathodes in tubes 92 and 93. Similarly the contacts of control relay 116 return to their tie-energized position, completing the circuit connecting the control grid to the cathode in tube 92, opening the contacts in the cathode circuits of tubes 92 and93 and closing the contact 116d, placing the motor field winding 36 in series with capacitor 109 and resistor 110 across the D. C. power supply. There is a momentary flow of D. C. through motor field winding 36 as a result of this circuit connection as capacitor 109 charges at a rate controlled by the impedance of resistor 110 and the motor field winding 36. The application of direct current to the field winding 36 as a result of the circuit connections just described, acts to instantly and effectively brake the motor M and eliminates any coasting or overtravel. Capacitor 109 discharges through contacts 1161) when contacts close on the next weld cycle. When time delay relay 121 times out its contacts 1210 open tie-energizing the gas and water solenoid valves and thereby interrupting the gas and water flow. The water fiow switch 131 then closes its contacts 131a. ()pening line switch 102 completely deenergizes the controls, and when the welding machine 37 is turned off the entire circuit is returned to the inoperative condition. g

It is obvious that other accessories and devices may be automatically operated by the above described circuit in the same manner and from the same or a similar power receptacle as the motorized carriage described herein. It is also abvious that additional circuitry can be added to that described to provide the operator with manual control of the apparatus and with optional starting sequences. Also such embellishments as limit switches etc., have not been described herein in the interest of maintaining maximum simplicity.

While only one embodiment of the invention has been a difference voltage, amplifying the diflerence voltage,

applying the amplified difference voltage across the con trol grids of grid controlled rectifier tubes in a full wave rectifier circuit, applying the output of said rectifier to a first field winding of a two phase induction motor, applying A. C. substantially 90 out of phase with the output of said rectifier to a second field winding of said motor wherebythe direction and speed of rotation of said motor will be determined by the sense and magnitude of said difference voltage, and advancing or retracting said electrode with respect to said workpiece in response to the rotation of said motor to automatically maintain the voltage across said arc gap at some predetermined value.

2. An automatic arc welding head comprising means for supporting an electrode in spaced relation to a workpiece to form an arc gap between said electrode and said ing a fixed reference voltage, means for comparing said fixed reference voltage with the voltage across said are gap, means for amplifying the difference between said reference voltage and said are voltage, a full wave grid controlled rectifier circuit, means for applying the amplified difference voltage across the control grids of the rectifier tubes in said rectifier circuit, means for applying the output of said rectifier to a first field Winding of said two phase induction motor, and means for energizing a second field winding of said motor with alternating current substantially out of phase with the output of said rectifier circuit.

3. An automatic arc welding head according to claim 2 in which a capacitor is connected in parallel with said first motor field winding to discharge through said first motor field winding and produce, with the conduction of one of said gridcontrolled rectifiers, alternating current in said motor field winding when the other of said grid controlled rectifiers is at or near cutoff.

4. An automatic arc welding head according to claim 2 in which a bias force tends to move the electrode toward the work including means to by-pass one of said rectifier tubes with a limited current flow to produce in said first motor fieldwinding a minimum excitation producing in said motor a torque in a direction urging said electrode away from said work.

5. An electric arc welding head having a two phase induction motor connected to move a welding electrode toward and away from a workpiece in response to variations in the voltage betweensaid electrode and said workpiece, means for energizing a first motor field winding from a full wave rectifier circuit, means for energizing a second motor field winding from an A. C. supply substantially 90 out of phase with the output of said rectifier circuit, and means for instantly stopping said motor by interrupting the operation of said rectifier circuit and passing a D. C. pulse of limited duration through said first motor. field winding to dynamically brake said motor.

6. An electric arc welding head according to claim 5 in which said means for instantly stopping said motor includes means for connecting said first motor field winding in a series circuit with a capacitor and a current limiting resistor, across a source of direct current.

References Cited in the file of this patent UNITED STATES PATENTS 2,008,411 Blankenbuehler et al. July 16, 1935 2,276,644 Blankenbuehler Mar. 17, 1942 2,460,990 Kratz et al. Feb. 8, 1949 2,500,341 Jacobson Mar. 14, 1950 2,527,893 Smith Oct. 31, 1950 2,529,111 Steinberger Nov. 7, 1950 2,691,715 Flora Oct. 12, 1954 

